The invention relates to coated metal elements in glass production, to a process for coating metal elements, particularly metal elements which on one side come in contact with molten glass, and to uses of such coated metal elements in the production and processing of glass.
In the glass-processing industry, containers, means of transport and tools for the handling of glass melts in the production and finishing areas are in many cases made of noble metals of the platinum group (platinum, palladium, rhodium, iridium) or of alloys of these metals with one another or with gold because of their heat-resistance and oxidation resistance. In systems for which this appears reasonable, the surfaces coming in contact with molten glass are coated with a metal of the indicated kind or are covered with a lining made of such a metal. An example of this is a feeder channel provided with a platinum lining which channel constitutes part of a glass-melting system and is usually needed for transporting liquid glass from a melting unit to a processing unit and to glass conditioning.
Molten glass always contains a certain amount of water stemming from the raw materials used for the glass melt, materials which bring the water bound therein into the manufacturing process. The atmosphere surrounding the glass melt and the water it contains, however, must also be taken into consideration. This is true particularly for water-rich furnace atmospheres in oxyfuel-fired melting tanks in which the water contained in the glass can be up to 50% higher than that in tanks that are not oxyfuel-fired.
Depending on the actually prevailing temperature, a certain percentage of the water contained in a glass melt dissociates in contact with a platinum metal forming hydrogen and oxygen. Another reason for water dissociation is based on electrolysis induced by an electric current flowing through the glass. An electric current is introduced into glass, for example, when the glass is heated with electrodes. Whereas hydrogen can diffuse through platinum metals, the oxygen generated in glass cannot. As a result of H2 diffusion and the attendant drop in H2 concentration in the glass, water dissociation reaches an equilibrium. The hydrogen that has diffused through the metal is carried away by the air. The oxygen accumulates at the interface with the platinum until the O2 concentration has exceeded the solubility limit at an O2 partial pressure of about 1 bar and O2-containing bubbles are formed. The bubbles grow as a result of SO2, N2, CO2 and other gases physically dissolved in the glass diffusing into them. They are then carried away by the glass stream and end up in the finished product. This can have a very deleterious effect on the quality of the glass products made.
This affects, in particular, borosilicate glasses, aluminosilicate glasses and glass ceramics. Among the borosilicate glasses, the neutral glasses of importance in pharmacy, industry and chemistry, but also many other borosilicate glasses not belonging to the group of neutral glasses have a particularly pronounced tendency to form O2 bubbles. Among the aluminosilicate glasses, particularly affected are those used for display applications and for lamps exposed to high heat. Moreover, all glasses not containing a sufficiently high concentration of polyvalent ions and which have not been adequately fined have a tendency to form O2 bubbles at platinum surfaces.
It is known from U.S. Pat. No. 5,785,726 that containers made of platinum or a platinum alloy can be protected from the formation of electrochemically generated O2 bubbles by flushing with a hydrogen-containing or water vapor-containing atmosphere.
It is known from WO 98/18731 to prevent bubble formation in the contact zone between the glass melt and platinum or molybdenum by providing H2 on the side facing away from the glass melt. The H2 partial pressure from the outside prevents H2 from diffusing from the melt through the platinum.
The known processes are expensive. They must be supervised and controlled. Disturbances in the control or regulation of the gas supply cause production outages.
In U.S. Pat. No. 3,109,045 is described a crucible made of molybdenum, tungsten, platinum, iron or a resistance alloy and used for melting and fining glass made from mixtures. The crucible is immersed into the liquid glass to prevent admission of air and is thus protected from atmospheric attack and oxidation. This is a method enabling the use of melting crucibles made of refractory metals such as, for example, molybdenum and tungsten which are not oxidation-resistant. Such a system is suitable for the melting of high-temperatured glasses and high-silica glasses with low thermal expansion. This system, however, can also be used for the melting of glasses having a low melting temperature.